To pass over a whole quickly, looking for what matters. A scan trades depth for coverage on purpose.
Latin scandere (to climb), used of counting a verse's metrical feet by climbing through them one by one; a quick pass trading depth for coverage grew from that older sense of stepping through something rhythmically.
A quick pass over a whole, looking for what matters and trading depth for coverage on purpose — the same climbing-word behind scale, applied to reading instead of measuring.